Introduction: Electromagnetic compatibility (EMC) refers to "the performance of a device, device, or system that enables it to function properly in its environment without causing strong electromagnetic interference with any other device in that environment."
  EMC problem is often a reason to restrict the export of China's electronic products, this article mainly discusses the source of EMI and some very specific suppression methods.
      Electromagnetic compatibility (EMC) is "the performance of a device, device, or system that enables it to function properly in its environment without causing strong electromagnetic interference with any other device in that environment" (IEEE C63.12-1987). For wireless transceivers, the use of discontinuous spectrum can partially achieve EMC performance, but many relevant examples also show that EMC is not always able to do so. For example, there is high-frequency interference between laptops and test equipment, between printers and desktop computers, and between cellular phones and medical devices, which is called electromagnetic interference (EMI).

EMC Problem Sources
  All electrical and electronic devices operate with intermittent or continuous voltage and current changes, sometimes at a fairly rapid rate, which causes electromagnetic energy to be generated within different frequencies or between one band, and the corresponding circuit will emit this energy into the surrounding environment.
      There are two ways for EMI to leave or enter a circuit: radiation and conduction. The signal radiation is leaked out through cracks, slots, openings or other gaps in the housing; The signal conduction leaves the housing by coupling to the power supply, signal and control lines, radiating freely in the open space, thereby creating interference.
      Many EMI suppression is achieved using a combination of shell shielding and gap shielding. Most of the time, these simple principles can help achieve EMI shielding: reduce interference at the source; By shielding, filtering or grounding to isolate the interference generating circuit and enhance the anti-interference ability of the sensitive circuit. EMI suppression, isolation, and low sensitivity should be the goal of all circuit designers, and these properties should be achieved early in the design phase.
      For design engineers, the use of shielding materials is an effective way to reduce EMI. A variety of shell shielding materials are now widely used, ranging from metal cans, thin metal sheets and foils to spraying coatings and coatings (such as conductive paint and zinc wire spraying) on conductive fabrics or coils. Whether it is metal or plastic coated with a conductive layer, once the designer has identified the material as the housing, the selection of the liner can begin.

Metal shielding efficiency
  The suitability of the shield can be evaluated using the shielding efficiency (SE), which is measured in decibels and calculated by the formula:
  SEdB=A+R+B
      Where A: absorption loss (dB) R: reflection loss (dB) B: correction factor (dB)(suitable for the presence of multiple reflections in the thin shield)
      A simple shield will reduce the intensity of the electromagnetic field generated to one-tenth of the original, that is, SE is equal to 20dB; Some occasions may require the field strength to be reduced to one hundred thousandth of the original, that is, SE is equal to 100dB.
      Absorption loss refers to the amount of energy loss when electromagnetic waves pass through the shield, and the absorption loss is calculated as follows:
      The AdB = 1.314 (f * mu sigma) 1/2 x t
      Where f: frequency (MHz) μ : magnetic permeability of copper σ : electrical conductivity of copper t: thickness of shield
      The size of the reflection loss (near field) depends on the nature of the source of the electromagnetic wave and the distance from the source. For a rod or linear transmitting antenna, the wave resistance increases as the distance from the wave source increases, and then decreases as the distance from the wave source increases, but the plane wave resistance does not change (constant 377).
      In contrast, if the wave source is a small coil, the magnetic field will dominate at this time, and the closer to the wave source, the lower the wave resistance. The wave resistance increases with the increase of the distance from the wave source, but when the distance exceeds one-sixth of the wavelength, the wave resistance does not change, and is constant at 377.
      The reflection loss varies with the ratio of wave resistance to shield impedance, so it depends not only on the type of wave, but also on the distance between the shield and the wave source. This applies to small shielded devices.
      The near-field reflection loss can be calculated as follows:
      R(electric)dB=321.8-(20×lg r)-(30×lg f)-[10×lg(μ/σ)]
       R(magnetic)dB=14.6+(20×lg r)+(10×lg f)+[10×lg(μ/σ)]
      Where r: the distance between the wave source and the shield.
      The last term of the SE formula is the correction factor B, which is calculated as follows:
      B = 20 lg [- exp (- 2 t/sigma)]
      This formula is only applicable to the near-magnetic field environment and the absorption loss is less than 10dB. Because the absorption efficiency of the shield is not high, the internal re-reflection will increase the energy passing through the other side of the shield, so the correction factor is a negative number, indicating the decrease in the shielding efficiency.

EMI suppression strategy
  Only materials with high magnetic permeability such as metals and iron can achieve high shielding efficiency at very low frequencies. The permeability of these materials will decrease with increasing frequency, in addition, if the initial magnetic field is strong, the permeability will be reduced, and the mechanical method of forming the shield into a specified shape will also reduce the permeability. To sum up, the selection of highly conductive magnetic materials for shielding is very complex, and it is usually necessary to seek solutions from EMI shielding material suppliers and relevant consulting institutions.
    Under high-frequency electric fields, the use of a thin layer of metal as a shell or lining material can achieve a good shielding effect, but the condition is that the shielding must be continuous, and the sensitive part is completely covered, without gaps or gaps (forming a Faraday cage). However, in practice, it is not possible to create a seamless and notched shield, because the shield is divided into multiple parts, so there are gaps that need to be joined, and usually have to drill holes in the shield to install the connection to the card or assembly component.
    The difficulty in designing a shield is that pores are inevitably created during the manufacturing process, and these pores are also needed during the operation of the equipment. Manufacturing, panel wiring, vents, external monitoring Windows, and panel mounting components all require holes in the shield, which greatly reduces shielding performance. Although grooves and gaps are unavoidable, it is beneficial to carefully consider the length of the grooves in relation to the operating frequency wavelength of the circuit in the shield design.
    The wavelength of any frequency electromagnetic wave is: wavelength (λ)= light speed (C)/ frequency (Hz)
    When the gap length is half the wavelength (cutoff frequency), the RF wave begins to decay at a rate of 20dB/10 octave (1/10 cutoff frequency) or 6dB/8 octave (1/2 cutoff frequency). Generally, the higher the RF emission frequency, the more serious the attenuation, because its wavelength is shorter. When it comes to the highest frequencies, any harmonics that may occur must be considered, but in practice only the first and second harmonics are considered.
    Once the frequency and intensity of RF radiation in the shield is known, the maximum permissible gap and groove of the shield can be calculated. For example, if you need to attenuate the radiation of 1GHz(wavelength of 300mm) by 26dB, then the gap of 150mm will start to attenuate, so when there is a gap less than 150mm, the 1GHz radiation will be attenuated. Therefore, for the 1GHz frequency, if you need to attenuate 20dB, the gap should be less than 15mm (1/10 of 150mm), when you need to attenuate 26dB, the gap should be less than 7.5mm (more than 1/2 of 15mm), and when you need to attenuate 32dB, the gap should be less than 3.75mm (more than 1/2 of 7.5mm).
    This attenuation effect can be achieved by using a suitable conductive liner to limit the gap size to the specified size.

Difficulty of shield design

  Since the joint will cause the permeability of the shield to decrease, the shielding efficiency will also decrease. It should be noted that the attenuation of radiation below the cutoff frequency depends only on the length to diameter ratio of the gap, for example, 100dB attenuation can be obtained when the length to diameter ratio is 3. When it is necessary to perforate, the waveguide characteristics of the holes on the thick shield can be used; Another way to achieve a higher length-to-diameter ratio is to attach a small metal shield, such as an appropriately sized liner. The above principles and their extension in the case of multiple cracks form the basis of the design of porous shield.
      Porous thin shielding layer: there are many examples of porous, such as ventilation holes on thin metal sheets, etc., when the holes are close to each other, the design must be carefully considered. The following is the formula for calculating the shielding efficiency in such cases:
      SE=[20lg (fc/o/σ)]-10lg n
      Where fc/o: cut-off frequency n: number of holes
      Note that this formula is only applicable when the hole spacing is smaller than the hole diameter, and can also be used to calculate the relevant shielding efficiency of metal braided nets.
    Joints and contacts: Electric welding, brazing or soldering is a common way of permanent fixation between sheets. The metal surface of the joint must be cleaned so that the joint can be completely filled with conductive metal. Fixing with screws or rivets is not recommended, because the low resistance contact state at the joint between the fasteners is not easy to maintain for a long time.
      The purpose of the conductive liner is to reduce the slot, hole or gap in the joint or joint so that the RF radiation will not be emitted. An EMI liner is a conductive medium used to fill gaps in the shield and provide continuous low-impedance contacts. Typically, an EMI liner provides a flexible connection between two conductors, allowing electricity from one conductor to flow to the other.
      The selection of EMI gaskets for sealing holes can refer to the following performance parameters:
      Shielding efficiency for a specific frequency range
      Installation method and sealing strength
      Compatibility with housing current and corrosion resistance to external environments

Operating temperature range
  Most commercial gaskets have sufficient shielding properties to meet EMC standards, the key is to properly design the gasket inside the shield.
      Gasket systems: An important factor to consider is compression, which creates a high conductivity between the gasket and the gasket. The poor conductivity between the gasket and the gasket will reduce the shielding efficiency, and if the joint is missing, there will be a thin slit to form a trough antenna, whose radiation wavelength is about 4 times smaller than the length of the gap.
      To ensure the conductivity, first ensure that the surface of the gasket is smooth, clean and treated as necessary to have good electrical conductivity, and these surfaces must be covered before joining; In addition, the shielding gasket material is also very important for this gasket to have continuous good adhesion. The compressibility of the conductive gasket can compensate for any irregularities in the gasket.
     All gaskets have an effective working minimum contact resistance, the designer can increase the compression of the gasket to reduce the contact resistance of multiple gaskets, of course, this will increase the strength of the seal, will make the shield become more curved. Most liners work well when compressed to 30% to 70% of their original thickness. Therefore, within the recommended minimum contact surface, the pressure between the two concave points should be sufficient to ensure good electrical conductivity between the gasket and the gasket.
     On the other hand, the pressure on the gasket should not be so large that the gasket is in an abnormal compression state, because this will cause the gasket contact failure and may produce electromagnetic leakage. The requirement to separate from the gasket is important to control the compression of the gasket within the manufacturer's recommended range, and this design needs to ensure that the gasket has sufficient hardness to avoid large bending between the gasket fasteners. In some cases, additional fasteners may be required to prevent the housing structure from bending.
     Compressibility is also an important characteristic of turning joints, such as in doors or inserts. If the gasket is easy to compress, then the shielding performance will decrease with each turn of the door, and the gasket needs a higher compression force to achieve the same shielding performance as the new gasket. This is unlikely to be possible in most cases, so a long-term EMI solution is needed.
     If the shield or gasket is made of plastic coated with a conductive layer, adding an EMI liner will not cause too many problems, but designers must consider that many liners will wear on conductive surfaces, and metal liners are usually more prone to wear on the coated surface. Over time this wear can reduce the shielding efficiency of the gasket joint and cause problems for subsequent manufacturers.
     If the shield or gasket structure is metal, then a liner can be added to cover the surface of the gasket before spraying the polishing material, using only conductive film and coil tape. If coil tape is used on both sides of the joint gasket, the EMI gasket can be tightened with mechanical firmware, such as a "Type C" gasket with plastic rivets or a pressure sensitive binder (PSA). The liner is installed on one side of the gasket to complete the EMI shielding.

Gaskets and accessories
  A wide range of shielding and gasket products are available today, including beryllium-copper joints, wire mesh (with or without an elastic core), wire mesh and directional wires embedded in rubber, conductive rubber, and polyurethane foam gaskets with a metallic coating. Most shielding material manufacturers can provide an estimate of the SE that a variety of liners can achieve, but keep in mind that SE is a relative value and depends on the porosity, liner size, liner compression ratio, and material composition. Gaskets come in a variety of shapes and can be used for a variety of specific applications, including wear, slide, and hinged situations. At present, many gaskets come with viscose or have fixed devices above the gasket, such as squeeze inserts, pin inserts or barb devices.
  Among the various types of padding, coated foam padding is one of the newest and most widely used products on the market. These gaskets can be made in a variety of shapes and thickness greater than 0.5mm or reduced to meet UL combustion and environmental seal standards. There is also a new type of liner, the environmental /EMI hybrid liner, which eliminates the need for a separate sealing material, thus reducing the cost and complexity of the shield. The outer coating of these liners is UV stable and resistant to moisture, wind and cleaning solvents, while the inner coating is metallized and has high electrical conductivity. Another recent innovation is the installation of a plastic clip on the EMI liner, which is more attractive to the market because of its lower weight, shorter assembly time and lower cost compared to the traditional pressed metal liner.

conclusion
  Equipment generally needs to be shielded, because there are some slots and gaps in the structure itself. The required shielding can be determined by some basic principles, but there is a difference between theory and reality. For example, the strength of the signal must also be considered when calculating the size and spacing of the liner at a certain frequency, as is the case when multiple processors are used in a device. Surface treatment and gasket design are key factors in maintaining long-term shielding to achieve EMC performance.